Several ongoing projects in the Nuclear Medicine Department use radiolabeled monoclonal antibodies for the diagnosis and treatment of malignant tumors. Currently, most of the monoclonal antibodies used for these studies are murine in origin. In response to the injection of these foreign proteins, patients develop anti-antibodies, commonly termed human anti-murine antibodies (HAMA). The goal of this project was to develop methods to measure the levels of these antibodies in patient sera and to define the frequency of occurrence of HAMA. Three assay methods for HAMA have been developed including a method that uses size exclusion HPLC. We have shown that 1) 50% of patients who receive a single injection of murine IgG develop HAMA, whereas 6% of those who receive a Fab fragment become HAMA positive. 2) The percentage of antibody held in complex by a patient's serum is directly related to the levels of HAMA as measured by protein A. 3) The clearance of injected murine antibody from circulation and the subsequent biodistribution of radiolabeled murine antibody are related to the HAMA-antibody complex levels. 4) The fraction of murine antibody bound in complexes can be reduced by using large doses of murine antibody, an approach that could be used clinically to reduce the effect of HAMA. Since HAMA development is common in patients receiving monoclonal antibodies, the study and monitoring of this problem will be ongoing.